Polarity protein Par3 controls B-cell receptor dynamics and antigen extraction at the immune synapse

Positioning centrioles and centrosomes

Centrosomes are the primary microtubule organizer in eukaryotic cells. In addition to shaping the intracellular microtubule network and the mitotic spindle, centrosomes are responsible for positioning cilia and flagella. To fulfill these diverse functions, centrosomes must be properly located within cells, which requires that they undergo intracellular transport. Importantly, centrosome mispositioning has been linked to ciliopathies, cancer, and infertility. The mechanisms by which centrosomes migrate are diverse and context dependent. In many cells, centrosomes move via indirect motor transport, whereby centrosomal microtubules engage anchored motor proteins that exert forces on those microtubules, resulting in centrosome movement. However, in some cases, centrosomes move via direct motor transport, whereby the centrosome or centriole functions as cargo that directly binds molecular motors which then walk on stationary microtubules. In this review, we summarize the mechanisms of centrosome motility and the consequences of centrosome mispositioning and identify key questions that remain to be addressed.

Endolysosomal vesicles at the center of B cell activation

The endolysosomal system specializes in degrading cellular components and is crucial to maintaining homeostasis and adapting rapidly to metabolic and environmental cues. Cells of the immune system exploit this network to process antigens or promote cell death by secreting lysosome-related vesicles. In B lymphocytes, lysosomes are harnessed to facilitate the extraction of antigens and to promote their processing into peptides for presentation to T cells, critical steps to mount protective high-affinity antibody responses. Intriguingly, lysosomal vesicles are now considered important signaling units within cells and also display secretory functions by releasing their content to the extracellular space. In this review, we focus on how B cells use pathways involved in the intracellular trafficking, secretion, and function of endolysosomes to promote adaptive immune responses. A basic understanding of such mechanisms poses an interesting frontier for the development of therapeutic strategies in the context of cancer and autoimmune diseases.

Medium-throughput image-based phenotypic siRNA screen to unveil the molecular basis of B cell polarization

Cell polarity is an essential and highly conserved process governing cell function. Cell polarization is generally triggered by an external signal that induces the relocation of the centrosome, thus defining the polarity axis of the cell. Here, we took advantage of B cells as a model to study cell polarity and perform a medium-throughput siRNA-based imaging screen to identify new molecular regulators of polarization. We first identified candidates based on a quantitative proteomic analysis of proteins differentially associated with the centrosome of resting non-polarized and stimulated polarized B cells. We then targeted 233 candidates in a siRNA screen and identified hits regulating the polarization of the centrosome and/or lysosomes in B cells upon stimulation. Our dataset of proteomics, images, and polarity indexes provides a valuable source of information for a broad community of scientists interested in the molecular mechanisms regulating cell polarity.

Polarity in immune cells

Immune cells are responsible for pathogen detection and elimination, as well as for signaling to other cells the presence of potential danger. In order to mount an efficient immune response, they need to move and search for a pathogen, interact with other cells, and diversify the population by asymmetric cell division. All these actions are regulated by cell polarity: cell polarity controls cell motility, which is crucial for scanning peripheral tissues to detect pathogens, and recruiting immune cells to sites of infection; immune cells, in particular lymphocytes, communicate with each other by a direct contact called immunological synapse, which entails a global polarization of the cell and plays a role in activating lymphocyte response; finally, immune cells divide asymmetrically from a precursor, generating a diversity of phenotypes and cell types among daughter cells, such as memory and effector cells. This review aims at providing an overview from both biology and physics perspectives of how cell polarity shapes the main immune cell functions.

Autophagy Induced by Toll-like Receptor Ligands Regulates Antigen Extraction and Presentation by B Cells

The engagement of B cells with surface-tethered antigens triggers the formation of an immune synapse (IS), where the local secretion of lysosomes can facilitate antigen uptake. Lysosomes intersect with other intracellular processes, such as Toll-like Receptor (TLR) signaling and autophagy coordinating immune responses. However, the crosstalk between these processes and antigen presentation remains unclear. Here, we show that TLR stimulation induces autophagy in B cells and decreases their capacity to extract and present immobilized antigens. We reveal that TLR stimulation restricts lysosome repositioning to the IS by triggering autophagy-dependent degradation of GEF-H1, a Rho GTP exchange factor required for stable lysosome recruitment at the synaptic membrane. GEF-H1 degradation is not observed in B cells that lack αV integrins and are deficient in TLR-induced autophagy. Accordingly, these cells show efficient antigen extraction in the presence of TLR stimulation, confirming the role of TLR-induced autophagy in limiting antigen extraction. Overall, our results suggest that resources associated with autophagy regulate TLR and BCR-dependent functions, which can finetune antigen uptake by B cells. This work helps to understand the mechanisms by which B cells are activated by surface-tethered antigens in contexts of subjacent inflammation before antigen recognition, such as sepsis.

Phenotyping polarization dynamics of immune cells using a lipid droplet-cell pairing microfluidic platform

The immune synapse is the tight contact zone between a lymphocyte and a cell presenting its cognate antigen. This structure serves as a signaling platform and entails a polarization of intracellular components necessary to the immunological function of the cell. While the surface properties of the presenting cell are known to control the formation of the synapse, their impact on polarization has not yet been studied. Using functional lipid droplets as tunable artificial presenting cells combined with a microfluidic pairing device, we simultaneously observe synchronized synapses and dynamically quantify polarization patterns of individual B cells. By assessing how ligand concentration, surface fluidity, and substrate rigidity impact lysosome polarization, we show that its onset and kinetics depend on the local antigen concentration at the synapse and on substrate rigidity. Our experimental system enables a fine phenotyping of monoclonal cell populations based on their synaptic readout.

B Cells Adapt Their Nuclear Morphology to Organize the Immune Synapse and Facilitate Antigen Extraction

Upon interaction with immobilized antigens, B cells form an immune synapse where actin remodeling and re-positioning of the microtubule-organizing center (MTOC) together with lysosomes can facilitate antigen extraction. B cells have restricted cytoplasmic space, mainly occupied by a large nucleus, yet the role of nuclear morphology in the formation of the immune synapse has not been addressed. Here we show that upon activation, B cells re-orientate and adapt the size of their nuclear groove facing the immune synapse, where the MTOC sits, and lysosomes accumulate. Silencing the nuclear envelope proteins Nesprin-1 and Sun-1 impairs nuclear reorientation towards the synapse and leads to defects in actin organization. Consequently, B cells are unable to internalize the BCR after antigen activation. Nesprin-1 and Sun-1-silenced B cells also fail to accumulate the tethering factor Exo70 at the center of the synaptic membrane and display defective lysosome positioning, impairing efficient antigen extraction at the immune synapse. Thus, changes in nuclear morphology and positioning emerge as critical regulatory steps to coordinate B cell activation.

B-cell receptor dependent phagocytosis and presentation of particulate antigen by chronic lymphocytic leukemia cells

Aim

T-helper cells could play an important role in the pathogenesis of chronic lymphocytic leukemia (CLL), a common B-cell neoplasm. Although CLL cells can present soluble antigens targeted from the B-cell receptor to T-helper cells via major histocompatibility complex (MHC) class II, antigens recognized by some CLL cells may be encountered in a particulate form. Here the ability of CLL cells to internalize and present anti-immunoglobulin M (IgM) beads as a model for the interaction of CLL cells with particulate antigens was investigated.

Methods

The effect of anti-IgM beads on antigen presentation pathways was analyzed using RNA-seq and internalization of anti-IgM beads by primary CLL cells was investigated using confocal microscopy and flow cytometry. Antigen presentation was investigated by analyzing activation of a T-cell line expressing a T-cell receptor specific for a peptide derived from mouse κ light chains after incubating CLL cells with a mouse κ light chain-containing anti-IgM monoclonal antibody. Kinase inhibitors were used to characterize the pathways mediating internalization and antigen presentation.

Results

Stimulation of surface IgM of CLL cells increased expression of the antigen presentation machinery and CLL cells were able to phagocytose anti-IgM beads. Internalization of anti-IgM beads was associated with MHC class II-restricted activation of cognate T-helper cells. Antigen presentation by CLL cells was dependent on activity of spleen tyrosine kinase (SYK) and phosphatidylinositol 3-kinase delta (PI3Kδ) but was unaffected by inhibitors of Bruton's tyrosine kinase (BTK).

Conclusions

CLL cells can internalize and present antigen from anti-IgM beads. This capacity of CLL cells may be particularly important for recruitment of T-cell help in vivo in response to particulate antigens.

Ecm29-Dependent Proteasome Localization Regulates Cytoskeleton Remodeling at the Immune Synapse

The formation of an immune synapse (IS) enables B cells to capture membrane-tethered antigens, where cortical actin cytoskeleton remodeling regulates cell spreading and depletion of F-actin at the centrosome promotes the recruitment of lysosomes to facilitate antigen extraction. How B cells regulate both pools of actin, remains poorly understood. We report here that decreased F-actin at the centrosome and IS relies on the distribution of the proteasome, regulated by Ecm29. Silencing Ecm29 decreases the proteasome pool associated to the centrosome of B cells and shifts its accumulation to the cell cortex and IS. Accordingly, Ecm29-silenced B cells display increased F-actin at the centrosome, impaired centrosome and lysosome repositioning to the IS and defective antigen extraction and presentation. Ecm29-silenced B cells, which accumulate higher levels of proteasome at the cell cortex, display decreased actin retrograde flow in lamellipodia and enhanced spreading responses. Our findings support a model where B the asymmetric distribution of the proteasome, mediated by Ecm29, coordinates actin dynamics at the centrosome and the IS, promoting lysosome recruitment and cell spreading.

Diacylglycerol kinase ζ promotes actin cytoskeleton remodeling and mechanical forces at the B cell immune synapse

Diacylglycerol kinases (DGKs) limit antigen receptor signaling in immune cells by consuming the second messenger diacylglycerol (DAG) to generate phosphatidic acid (PA). Here, we showed that DGKζ promotes lymphocyte function-associated antigen 1 (LFA-1)-mediated adhesion and F-actin generation at the immune synapse of B cells with antigen-presenting cells (APCs), mostly in a PA-dependent manner. Measurement of single-cell mechanical force generation indicated that DGKζ-deficient B cells exerted lower forces at the immune synapse than did wild-type B cells. Nonmuscle myosin activation and translocation of the microtubule-organizing center (MTOC) to the immune synapse were also impaired in DGKζ-deficient B cells. These functional defects correlated with the decreased ability of B cells to present antigen and activate T cells in vitro. The in vivo germinal center response of DGKζ-deficient B cells was also reduced compared with that of wild-type B cells, indicating that loss of DGKζ in B cells impaired T cell help. Together, our data suggest that DGKζ shapes B cell responses by regulating actin remodeling, force generation, and antigen uptake-related events at the immune synapse. Hence, an appropriate balance in the amounts of DAG and PA is required for optimal B cell function.

“If you please… draw me a cell”. Insights from immune cells

Studies in recent years have shed light on the particular features of cytoskeleton dynamics in immune cells, challenging the classical picture drawn from typical adherent cell lines. New mechanisms linking the dynamics of the membrane–cytoskeleton interface to the mechanical properties of immune cells have been uncovered and shown to be essential for immune surveillance functions. In this Essay, we discuss these features, and propose immune cells as a new playground for cell biologists who try to understand how cells adapt to different microenvironments to fulfil their functions efficiently.

Actomyosin-driven force patterning controls endocytosis at the immune synapse

An important channel of cell-to-cell communication is direct contact. The immune synapse is a paradigmatic example of such type of interaction: it forms upon engagement of antigen receptors in lymphocytes by antigen-presenting cells and allows the local exchange of molecules and information. Although mechanics has been shown to play an important role in this process, how forces organize and impact on synapse function is unknown. We find that mechanical forces are spatio-temporally patterned at the immune synapse: global pulsatile myosin II-driven tangential forces are observed at the synapse periphery while localised forces generated by invadosome-like F-actin protrusions are detected at its centre. Noticeably, we observe that these force-producing actin protrusions constitute the main site of antigen extraction and endocytosis and require myosin II contractility to form. The interplay between global and local forces dictated by the organization of the actomyosin cytoskeleton therefore controls endocytosis at the immune synapse.

The immune synapse promotes cellular information exchange but the role of biophysical forces in synapse function is unclear. Here, the authors show that B cells exert two types of forces, a centripetal myosin II-driven force and a central actin protrusive force at the site of antigen extraction.

The exocyst controls lysosome secretion and antigen extraction at the immune synapse of B cells

BCR engagement enhances microtubule stability, which triggers the mobilization of Exo70 from the centrosome to the immune synapse. BCR engagement activates GEF-H1, which promotes exocyst assembly required for the docking and secretion of lysosomes, facilitating the extraction of surface-tethered antigens.

B lymphocytes capture antigens from the surface of presenting cells by forming an immune synapse. Local secretion of lysosomes, which are guided to the synaptic membrane by centrosome repositioning, can facilitate the extraction of immobilized antigens. However, the molecular basis underlying their delivery to precise domains of the plasma membrane remains elusive. Here we show that microtubule stabilization, triggered by engagement of the B cell receptor, acts as a cue to release centrosome-associated Exo70, which is redistributed to the immune synapse. This process is coupled to the recruitment and activation of GEF-H1, which is required for assembly of the exocyst complex, used to promote tethering and fusion of lysosomes at the immune synapse. B cells silenced for GEF-H1 or Exo70 display defective lysosome secretion, which results in impaired antigen extraction and presentation. Thus, centrosome repositioning coupled to changes in microtubule stability orchestrates the spatial-temporal distribution of the exocyst complex to promote polarized lysosome secretion at the immune synapse.

Proteasome Dependent Actin Remodeling Facilitates Antigen Extraction at the Immune Synapse of B Cells

Engagement of the B cell receptor (BCR) with surface-tethered antigens leads to the formation of an immune synapse (IS), where cell signaling and antigen uptake are tightly coordinated. Centrosome re-orientation to the immune synapse has emerged as a critical regulatory step to guide the local recruitment and secretion of lysosomes, which can facilitate the extraction of immobilized antigens. This process is coupled to actin remodeling at the centrosome and at the immune synapse, which is crucial to promote cell polarity. How B cells balance both pools of actin cytoskeleton to achieve a polarized phenotype during the formation of an immune synapse is not fully understood. Here, we reveal that B cells rely on proteasome activity to achieve this task. The proteasome is a multi-catalytic protease that degrades cytosolic and nuclear proteins and its dysfunction is associated with diseases, such as cancer and autoimmunity. Our results show that resting B cells contain an active proteasome pool at the centrosome, which is required for efficient actin clearance at this level. As a result of proteasome inhibition, activated B cells do not deplete actin at the centrosome and are unable to separate the centrosome from the nucleus and thus display impaired polarity. Consequently, lysosome recruitment to the immune synapse, antigen extraction and presentation are severely compromised in B cells with diminished proteasome activity. Additionally, we found that proteasome inhibition leads to impaired actin remodeling at the immune synapse, where B cells display defective spreading responses and distribution of key signaling molecules at the synaptic membrane. Overall, our results reveal a new role for the proteasome in regulating the immune synapse of B cells, where the intracellular compartmentalization of proteasome activity controls cytoskeleton remodeling between the centrosome and synapse, with functional repercussions in antigen extraction and presentation.

Positioning of the Centrosome and Golgi Complex

For over a century, the centrosome has been an organelle more easily tracked than understood, and the study of its peregrinations within the cell remains a chief underpinning of its functional investigation. Increasing attention and new approaches have been brought to bear on mechanisms that control centrosome localization in the context of cleavage plane determination, ciliogenesis, directional migration, and immunological synapse formation, among other cellular and developmental processes. The Golgi complex, often linked with the centrosome, presents a contrasting case of a pleiomorphic organelle for which functional studies advanced somewhat more rapidly than positional tracking. However, Golgi orientation and distribution has emerged as an area of considerable interest with respect to polarized cellular function. This chapter will review our current understanding of the mechanism and significance of the positioning of these organelles.

ATG5 is required for B cell polarization and presentation of particulate antigens

The involvement of macroautophagy/autophagy proteins in B-cell receptor (BCR) trafficking, although suspected, is not well understood. We show that ATG5 (autophagy related 5) contributes to BCR polarization after stimulation and internalization into LAMP1 (lysosomal-associated membrane protein 1)⁺ and major histocompatibility complex class II (MHC-II)⁺ compartments. BCR polarization is crucial in the context of immobilized antigen processing. Moreover, antigen presentation to cognate T cells is decreased in the absence of ATG5 when the model antigen OVAL/ovalbumin is provided in an immobilized form in contrast to the normal presentation of soluble OVAL. We further show that ATG5 is required for centrosome polarization and actin nucleation in the immune synapse area. This event is accompanied by an increased interaction between ATG16L1 (autophagy related 16-like 1 [S. cerevisiae]) and the microtubule-organizing center-associated protein PCM1 (pericentriolar material 1). In the human B cell line BJAB, PCM1 is required for BCR polarization after stimulation. We thus propose that the ATG12 (autophagy related 12)-ATG5-ATG16L1 complex under BCR stimulation allows its interaction with PCM1 and consequently facilitates centrosome relocalization to the immune synapse, optimizing the presentation of particulate antigens. Abbreviations: ACTB: actin beta; ACTR2/3: ARP2/3 actin-related protein 2/3; APC: antigen-presenting cells; ATG: autophagy-related; BCR: B cell receptor; BECN1/Beclin 1: beclin 1, autophagy related; CDC42: cell division cycle 42; Cr2: complement receptor 2; CSFE: carboxyfluorescein succinimidyl ester; DAPI: 4',6-diamidino-2-phenylindole dihydrochloride; EEA1: early endosome antigen 1; ELISA: enzyme-linked immunosorbent assay; FITC: fluorescein isothyocyanate; GC: germinal center; GJA1/CX3: gap junction protein, alpha 1; Ig: immunoglobulin; LAMP1: lysosomal-associated membrane protein 1; LAP: LC3-associated phagocytosis; LM: littermate; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MAPK/ERK: mitogen activated protein kinase; MHC-II: major histocompatibility complex class II; MIIC: MHC class II compartment; OVAL: ovalbumin; PBS: phosphate-buffered saline; PCM1: pericentriolar material 1; PtdIns3K: phosphatidylinositol 3-kinase; PTPRC/CD45RB/B220; Protein tyrosine phosphatase, receptor type, C; SYK: spleen tyrosine kinase; TBS: Tris-buffered saline; TCR: T cell receptor; ULK1: unc-51 like kinase 1.

Tuning B cell responses to antigens by cell polarity and membrane trafficking

The capacity of B lymphocytes to produce specific antibodies, particularly broadly neutralizing antibodies that provide immunity to viral pathogens has positioned them as valuable therapeutic targets for immunomodulation. To become competent as antibody secreting cells, B cells undergo a series of activation steps, which are triggered by the recognition of antigens frequently displayed on the surface of other presenting cells. Such antigens elicit the formation of an immune synapse (IS), where local cytoskeleton rearrangements coupled to mechanical forces and membrane trafficking orchestrate the extraction and processing of antigens in B cells. In this review, we discuss the molecular mechanisms that regulate polarized membrane trafficking and mechanical properties of the immune synapse, as well as the potential extracellular cues from the environment, which may impact the ability of B cells to sense and acquire antigens at the immune synapse. An integrated view of the diverse cellular mechanisms that shape the immune synapse will provide a better understanding on how B cells are efficiently activated.

The Rap2c GTPase facilitates B cell receptor-induced reorientation of the microtubule-organizing center

When B lymphocytes encounter antigen-bearing surfaces, B-cell receptor (BCR) signaling initiates remodeling of the F-actin network and reorientation of the microtubule-organizing center (MTOC) towards the antigen contact site. We have previously shown that the Rap1 GTPase, an evolutionarily conserved regulator of cell polarity, is essential for these processes and that Rap1-regulated actin remodeling is required for MTOC polarization. The role of Rap2 proteins in establishing cell polarity is not well understood. We now show that depleting Rap2c, the only Rap2 isoform expressed in the A20 B-cell line, impairs BCR-induced MTOC reorientation as well as the actin remodeling that supports MTOC polarization. Thus Rap1 and Rap2 proteins may have similar but non-redundant functions in coupling the BCR to MTOC polarization.

Vamp-7-dependent secretion at the immune synapse regulates antigen extraction and presentation in B-lymphocytes

Recognition of surface-tethered antigens (Ags) by B-cells leads to the formation of an immune synapse that promotes Ag uptake for presentation onto MHC-II molecules. Extraction of immobilized Ags at the immune synapse of B-cells relies on the local secretion of lysosomes, which are recruited to the Ag contact site by polarization of their microtubule network. Although conserved polarity proteins have been implicated in coordinating cytoskeleton remodeling with lysosome trafficking, the cellular machinery associated with lysosomal vesicles that regulates their docking and secretion at the synaptic interface has not been defined. Here we show that the v-SNARE protein Vamp-7 is associated with Lamp-1⁺ lysosomal vesicles, which are recruited and docked at the center of the immune synapse of B-cells. A decrease in Vamp-7 expression does not alter lysosome transport to the synaptic interface but impairs their local secretion, a defect that compromises the ability of B-cells to extract, process, and present immobilized Ag. Thus our results reveal that B-cells rely on the SNARE protein Vamp-7 to promote the local exocytosis of lysosomes at the immune synapse, which is required for efficient Ag extraction and presentation.

The Rap1-cofilin-1 pathway coordinates actin reorganization and MTOC polarization at the B cell immune synapse

B cells that bind antigens displayed on antigen-presenting cells (APCs) form an immune synapse, a polarized cellular structure that optimizes the dual functions of the B cell receptor (BCR), signal transduction and antigen internalization. Immune synapse formation involves polarization of the microtubule-organizing center (MTOC) towards the APC. We now show that BCR-induced MTOC polarization requires the Rap1 GTPase (which has two isoforms, Rap1a and Rap1b), an evolutionarily conserved regulator of cell polarity, as well as cofilin-1, an actin-severing protein that is regulated by Rap1. MTOC reorientation towards the antigen contact site correlated strongly with cofilin-1-dependent actin reorganization and cell spreading. We also show that BCR-induced MTOC polarization requires the dynein motor protein as well as IQGAP1, a scaffolding protein that can link the actin and microtubule cytoskeletons. At the periphery of the immune synapse, IQGAP1 associates closely with F-actin structures and with the microtubule plus-end-binding protein CLIP-170 (also known as CLIP1). Moreover, the accumulation of IQGAP1 at the antigen contact site depends on F-actin reorganization that is controlled by Rap1 and cofilin-1. Thus the Rap1-cofilin-1 pathway coordinates actin and microtubule organization at the immune synapse.

To use or not to use the force: How B lymphocytes extract surface-tethered antigens

Pierobon and Lennon-Duménil highlight recent findings on how the mechanical properties of membranes affect uptake of surface-tethered antigen by B lymphocytes.

Using an exquisite cell imaging approach based on DNA nanosensors, Spillane and Tolar (2016. J. Cell Biol.https://doi.org/10.1083/jcb.201607064) explore how the physical properties of antigen-presenting cell surfaces affect how B cells internalize surface-tethered antigens. Soft and flexible surfaces promote mechanical force-mediated antigen extraction, whereas stiff surfaces lead to enzyme-mediated antigen release before subsequent internalization.

B cell antigen extraction is regulated by physical properties of antigen-presenting cells

Antibody production and affinity maturation are driven by B cell extraction and internalization of antigen from immune synapses. However, the extraction mechanism remains poorly understood. Here we develop DNA-based nanosensors to interrogate two previously proposed mechanisms, enzymatic liberation and mechanical force. Using antigens presented by either artificial substrates or live cells, we show that B cells primarily use force-dependent extraction and resort to enzymatic liberation only if mechanical forces fail to retrieve antigen. The use of mechanical forces renders antigen extraction sensitive to the physical properties of the presenting cells. We show that follicular dendritic cells are stiff cells that promote strong B cell pulling forces and stringent affinity discrimination. In contrast, dendritic cells are soft and promote acquisition of low-affinity antigens through low forces. Thus, the mechanical properties of B cell synapses regulate antigen extraction, suggesting that distinct properties of presenting cells support different stages of B cell responses.

TrackMate: An open and extensible platform for single-particle tracking

We present TrackMate, an open source Fiji plugin for the automated, semi-automated, and manual tracking of single-particles. It offers a versatile and modular solution that works out of the box for end users, through a simple and intuitive user interface. It is also easily scriptable and adaptable, operating equally well on 1D over time, 2D over time, 3D over time, or other single and multi-channel image variants. TrackMate provides several visualization and analysis tools that aid in assessing the relevance of results. The utility of TrackMate is further enhanced through its ability to be readily customized to meet specific tracking problems. TrackMate is an extensible platform where developers can easily write their own detection, particle linking, visualization or analysis algorithms within the TrackMate environment. This evolving framework provides researchers with the opportunity to quickly develop and optimize new algorithms based on existing TrackMate modules without the need of having to write de novo user interfaces, including visualization, analysis and exporting tools. The current capabilities of TrackMate are presented in the context of three different biological problems. First, we perform Caenorhabditis-elegans lineage analysis to assess how light-induced damage during imaging impairs its early development. Our TrackMate-based lineage analysis indicates the lack of a cell-specific light-sensitive mechanism. Second, we investigate the recruitment of NEMO (NF-κB essential modulator) clusters in fibroblasts after stimulation by the cytokine IL-1 and show that photodamage can generate artifacts in the shape of TrackMate characterized movements that confuse motility analysis. Finally, we validate the use of TrackMate for quantitative lifetime analysis of clathrin-mediated endocytosis in plant cells.

Actin nucleation at the centrosome controls lymphocyte polarity

Cell polarity is required for the functional specialization of many cell types including lymphocytes. A hallmark of cell polarity is the reorientation of the centrosome that allows repositioning of organelles and vesicles in an asymmetric fashion. The mechanisms underlying centrosome polarization are not fully understood. Here we found that in resting lymphocytes, centrosome-associated Arp2/3 locally nucleates F-actin, which is needed for centrosome tethering to the nucleus via the LINC complex. Upon lymphocyte activation, Arp2/3 is partially depleted from the centrosome as a result of its recruitment to the immune synapse. This leads to a reduction in F-actin nucleation at the centrosome and thereby allows its detachment from the nucleus and polarization to the synapse. Therefore, F-actin nucleation at the centrosome—regulated by the availability of the Arp2/3 complex—determines its capacity to polarize in response to external stimuli.

Cell polarity is marked by re-orientation of the centrosome, but the mechanisms governing centrosome polarization are poorly understood. Here Obino et al. show that in lymphocytes centrosome-associated Arp2/3 nucleates actin that tethers the centrosome to the nucleus; activation depletes Arp2/3 from the centrosome and frees it from the nucleus.

Polarity and asymmetric cell division in the control of lymphocyte fate decisions and function

Polarity is important in several lymphocyte processes including lymphocyte migration, formation of the immunological synapse, and asymmetric cell division (ACD). While lymphocyte migration and immunological synapse formation are relatively well understood, the role of lymphocyte ACD is less clear. Recent advances in measuring polarity enable more robust analyses of asymmetric cell division. Use of these new methods has produced crucial quantification of ACD at precise phases of lymphocyte development and activation. These developments are leading to a better understanding of the drivers of fate choice during lymphocyte activation and provide a context within which to explain the effects of ACD.

Lethal giant larvae-1 deficiency enhances the CD8(+) effector T-cell response to antigen challenge in vivo

Lethal giant larvae-1 (Lgl-1) is an evolutionary conserved protein that regulates cell polarity in diverse lineages; however, the role of Lgl-1 in the polarity and function of immune cells remains to be elucidated. To assess the role of Lgl-1 in T cells, we generated chimeric mice with a hematopoietic system deficient for Lgl-1. Lgl-1 deficiency did not impair the activation or function of peripheral CD8(+) T cells in response to antigen presentation in vitro, but did skew effector and memory T-cell differentiation. When challenged with antigen-expressing virus or tumor, Lgl-1-deficient mice displayed altered T-cell responses. This manifested in a stronger antiviral and antitumor effector CD8(+) T-cell response, the latter resulting in enhanced control of MC38-OVA tumors. These results reveal a novel role for Lgl-1 in the regulation of virus-specific T-cell responses and antitumor immunity.

B Cells use Conserved Polarity Cues to Regulate Their Antigen Processing and Presentation Functions

The ability of B cells to produce high-affinity antibodies and to establish immunological memory in response to a wide range of pathogenic antigens is an essential part of the adaptive immune response. The initial step that triggers a humoral immune response involves the acquisition of antigens by B cells via their surface immunoglobulin, the B cell receptor (BCR). BCR-engaged antigens are transported into specialized lysosomal compartments where proteolysis and production of MHC class II-peptide complexes occur, a process referred to as antigen processing. Expression of MHC class II complexes at the B cell surface allows them to interact with T cells and to receive their help to become fully activated. In this review, we describe how B cells rely on conserved cell polarity mechanisms to coordinate local proteolytic secretion and mechanical forces at the B cell synapse enabling them to efficiently acquire and present extracellular antigens. We foresee that the mechanisms that dictate B cell activation can be used to tune B cell responses in the context of autoimmune diseases and cancer.